Window regulator existing technology typically has a metal rail (or two) that provides a guide and surface for the lifter plate to slide up and down along—this being the part of the window regulator that connects to the glass. Generally, the rail is made of metal to overcome some of the high loads applied by the motor and the varying conditions in service. Loading under hot and humid conditions tends to be the worst strain on the window regulator as a system.
Today's trend is to consider light weight solutions. To replace a metal rail with a lighter material such as plastic, one must consider the worst case loading conditions. Typically, the worst case loading conditions are found in high heat (+80 C) and high humidity (100% RH). These types of test environments are used to simulate areas of the world that have long term high heat and humidity such as Florida in the southern US.
These loading conditions complicate the design of non-metal rails due to the difficulty in designing with those materials to accommodate the loads that will be imposed. For instance, materials such as plastic may not be at their ideal strength conditions under hot and humid conditions, and the structure required to support the necessary loads in such environments can lead to an excessive amount of supporting plastic structure to overcome these loading conditions. Potentially, the amount of supporting plastic structure can raise costs and make it difficult to package the design—big and bulky means hard to package in a typical car door environment.